Genetics Home > Genetic Cause of Klinefelter Syndrome
The genetic cause of Klinefelter syndrome is the presence of one or more extra copies of the X chromosome in a male's cells. Extra copies of genes on the X chromosome interfere with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone.
Most often, the genetic cause of Klinefelter syndrome is a single extra copy of the X chromosome, for a total of 47 chromosomes per cell. Males normally have one X chromosome and one Y chromosome in each cell (46 ,XY), but males with Klinefelter syndrome have two X chromosomes and one Y chromosome (47,XXY).
Variations of XXY Chromosomes
Occasionally, variations of the XXY chromosome count may occur. Variants of Klinefelter syndrome are caused by several extra copies of the X chromosome, or extra copies of both the X and Y chromosomes, in all of the body's cells. The signs and symptoms of these conditions typically become more severe as the number of X chromosomes increases.
The most common XXY variation is the XY/XXY mosaic. In this variation, some of the cells in the male's body have an additional X chromosome and the rest have the normal XY chromosome count. The percentage of cells containing the extra chromosome varies from case to case. In some instances, XY/XXY mosaics may have enough normally functioning cells in the testes to allow them to father children.
A few instances of males having two, or even three, additional X chromosomes have also been reported in the medical literature. In these individuals, the classic features of Klinefelter syndrome may be exaggerated, with low I.Q. or moderate to severe mental retardation also occurring.
In rare instances, an individual may possess both an additional X and an additional Y chromosome. The medical literature describes XXYY males as having slight to moderate mental retardation. They may sometimes be aggressive or even violent. Klinefelter research scientists admit, however, that because these cases are so rare, not much is known about them.